In my earlier U.S. Pat. No. 4,267,281, I disclosed the preparation of controlled release pesticide-polymers by reacting a polymer having pendant groups containing a reactive hydrogen with an isocyanate derivative of a pesticide. The isocyanate derivatives described in that patent were prepared by reacting an amino containing pesticide with phosgene under suitable reaction conditions as described in the literature. (See R. Morrison and R. Boyd, Organic Chemistry, 2nd Ed. 1966, pp. 926-927 and R. Fuson, Reactions of Organic Compounds, 1964, p. 337). The patent also disclosed that the isocyanate derivatives could be prepared from the corresponding amides using a Hoffman rearrangement. (See. D. Grant and G. Hammond, Organic Chemistry, 2nd Ed. 1964, pp. 90 and 304).
A pesticide of special interest and for which there could be extensive use in a controlled-release form is the pre- and post-emergence herbicide 4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H) one, which is commonly known as metribuzin.
It is difficult to prepare the isocyanate derivative of some pesticides, such as metribuzin, in good yields using prior art methods. When metribuzin and phosgene are reacted at temperatures of about 66.degree. C., as suggested in the literature and my earlier patent, very little metribuzin isocyanate is produced. Reaction at higher temperature and/or in the presence of tertiary amines has no effect in increasing the yield of isocyanate and can lead to the production of competing products. The major products produced at those temperatures are the cyclic trimer, linear polyurets, or a mixture of the two, which are formed by competing reactions. Apparently, at temperatures of about 66.degree. C. enough activation energy is present to cause further reaction of any metribuzin isocyanate which forms to yield the cyclic trimer and/or polyurets. The rate of such reactions competes with that of isocyanate formation. As a result, only a low, steady-state concentration of the isocyanate derivative is obtained.
There is a need for an improved method of preparing controlled release pesticide-polymers which does not require the use of isocyanate derivatives of pesticides, which derivatives may be difficult to obtain in good yields.